When the pressure ratio P2/P1 between the pressure P1 upstream of the orifice and the pressure P2 downstream of the orifice is equal to or lower than the pressure ratio at which the critical expansion conditions of a gas are established, the orifice-passing gas flows at the speed of sound, and a variation in the pressure P2 downstream of the orifice is not transmitted to the upstream side. As a result, when the orifice has a fixed diameter, regardless of the kind of gas, the flow rate of the gas passing through the orifice plate changes in direct proportion to the gas pressure P1 upstream of the orifice.
Meanwhile, utilizing such characteristics of an orifice, a large number of fluid flow controller using an orifice have been developed.
FIG. 13 shows an example of the configuration of the pressure-controlled flow controller using an orifice previously disclosed by the present inventors. The flow controller 21 includes a control valve 22, a pressure detector 23, a temperature detector 24, an orifice 25, an arithmetic and control unit 26, amplifiers 27a and 27b, A/D converters 28a and 28b, and the like (JP-A-8-338546).
The fluid pressure P1 upstream of the orifice 25 is detected by the pressure detector 23 and input to the arithmetic and control unit 26. In the arithmetic and control unit 26, the flow rate Qc is calculated using the arithmetic expression Qc=KP1, while Qc is compared with the flow command value Qs, and the control signal Qy corresponding to the difference between the two, Qc−Qs, is input to an actuator 30 of the control valve 22.
In addition, the control valve 22 is opened/closed by the control signal Qy in the direction that the difference Qc−Qs approaches zero, whereby the flow rate downstream of the orifice 25 is constantly maintained at the set flow rate (flow command value) Qs.
Further, the orifice 25 is formed by making a small hole having an inner diameter of 0.01 to 0.20 mm in a metal plate having a thickness of 0.02 to 0.20 mm by pressing, electric discharge machining, or etching. The diameter of the orifice is appropriately selected according to the required gas flow rate to be controlled.
Furthermore, although electric discharge machining or etching is generally used to form an orifice 25, in some cases, the orifice is formed by so-called cutting using a drill in order to reduce the processing cost (JP-A-11-117915).
FIG. 14 shows the flow control characteristics of the pressure-controlled flow controller of FIG. 13 in the case where the gas is nitrogen gas, and shows the case where the pressure downstream of the orifice 25 is atmospheric pressure.
As clearly shown in FIG. 14, at a range where the pressure P1 upstream side exceeds about twice the pressure P2 on the downstream side, the relation between the flow rate Qc and P1 is kept linear, and Qc is in direct proportion to the pressure P1 upstream of the orifice. Thus, by automatically controlling the pressure P1 upstream of the orifice, the feedback control of the flow rate through the orifice is possible. Furthermore, in FIG. 14, A shows the flow control characteristics in the case where the diameter of the orifice is φ0.37 mm, while B is in the case of 0.20φ.
As clearly shown in FIG. 14, when the lines A and B are both within a range of P2<0.5P1 (i.e., P2/P1<0.5), the linearity is well maintained, and the flow rate can be precisely controlled by regulating P1.
However, it is known that the actual lower limit of P1 at which the critical expansion conditions of a gas are established (P2/P1<0.5 or P1/P2>2) (i.e., the lower limit of P1 at which the linearity is maintained) slightly changes with the inner diameter of the orifice, and the greater the diameter of an orifice is, the smaller the range of P2/P1 at which critical expansion conditions are established tends to be. That is, when P2 is constant, the lower limit of the control range of P1 increases.
Specifically, with an increase in the diameter of an orifice, the critical pressure ratio P2/P1<0.5 decreases to about P2/P1<0.45, and when P2 is constant, the lower limit of the control range of P1 increases, resulting in a smaller control range of P1.
In other words, when the diameter of an orifice increases with an increase in the controlled flow rate of a flow controller, the control range of the critical pressure ratio P2/P1 becomes smaller. This results in various inconveniences in the case where, for example, the gas is supplied to a vacuum chamber of a semiconductor manufacturing system.
As mentioned above, the conventional pressure-type flow controller using an orifice plate provided with one orifice has the drawback that the pressure ratio P2/P1 at which critical expansion conditions are established varies with an increase in the diameter of the orifice, resulting in the variation of the flow (pressure) control range. Therefore, in the technical field of pressure-controlled flow controllers applied to semiconductor manufacturing systems, there has been a strong demand for the advent an orifice plate for flow control, in which even when the diameter of an orifice changes, no variation occurs in the actual pressure ratio P2/P1 at which critical expansion conditions are established; and also a pressure-controlled flow controller using the same.